remove std::pair support

Code from https://stackoverflow.com/a/44503794/266378 which is hopefully working with MSVC.

Issue #593 Fix the arithmetic operators in the iterator and reverse iterator

Add pair support, fix CompatibleObject conversions (fixes #600)

Instead of calling CompatibleObjectType iterator-range constructor,
first convert json::value_type to CompatibleObjectType::value_type

#550 Fix iterator related compiling issues for Intel icc

This reverts commit c4ab8f82.

I totally forgot about byte order marks in this scenario.

Change the definition of the operator override of reverse iterator to using the result of the base class directly